Task-shifting of dental hygiene therapy to community health workers and untreated caries trajectories in rural Rwandan health centres: a difference-in-differences estimation of delegation-intensity thresholds

Rural Rwandan health centres face a persistent shortage of dental hygienists, and policy interest has turned to task-shifting preventive oral care to community health workers (CHWs). The framework specifies delegation intensity as a continuous treatment, defines the threshold as the point where the marginal effect on untreated caries prevalence changes sign, and derives the parallel-trends assumption required for credible estimation. Using a demonstration dataset of 15 country-level observations from the Demographic and Health Surveys Program indicator 'Family planning messages at community meeting or event' (2021–2025; mean 29.733, SD 17.495), the paper shows how the estimator recovers a threshold and how that threshold is sensitive to confounding by health-centre capacity. The contribution is methodological: a transparent, testable definition of delegation-intensity thresholds that can be applied to real caries data when available, together with an explicit account of the assumptions under which such thresholds are identified. Threshold estimates are only as credible as the parallel-trends assumption, and that delegation intensity should be modelled continuously rather than as a binary treatment.